1. Field of the Invention
The present invention relates to an asymmetrical bis-hydroxyenamine compound, an electrophotographic photoreceptor comprising the asymmetrical bis-hydroxyenamine compound, and an image forming apparatus equipped with the electrophotographic photoreceptor.
2. Description of the Related Art
An electrophotographic image forming apparatus for forming an image through electrophotography (hereinafter referred to also as “electrophotographic apparatus”) is much used as copying machines, printers, facsimile apparatus, etc. In the electrophotographic apparatus, an image is formed according to an electrophotographic process as follows. First, the photosensitive layer of an electrophotographic photoreceptor (this may be hereinafter simply referred to also as “photoreceptor”) is charged and then exposed to light to thereby form an electrostatic latent image. The thus-formed electrostatic latent image is developed to form a toner image. Thus formed, the toner image is transferred onto a transfer material such as recording paper, and then fixed. According to the process, a desired image is formed on a transfer material.
Recently, electrophotography is utilized not only in the field of printing machines but also in other fields of printing plate materials, slide films, microfilms and others for which silver salt photography has heretofore been used, and in addition, it is further applied to high-speed printers using a light source of laser, light-emitting diode (abbreviated as LED), cathode ray tube (abbreviated as CRT) or the like. With the expansion of the application field of electrophotography, requirements for electrophotographic photoreceptors are being high-leveled and broadened.
As the electrophotographic photoreceptor, conventionally broadly used is an inorganic photoreceptor equipped with a photosensitive layer comprising, as the main ingredient thereof, an inorganic photoconductive material such as selenium, zinc oxide or cadmium sulfide. The inorganic photoreceptor has basic properties as a photoreceptor in some degree, but has some drawbacks in that the film formation for the photosensitive layer is difficult, the plasticity is not good and the production costs are high. In addition, in general, inorganic photoconductive materials are highly toxic, and are therefore greatly limited in point of their production and use.
As described above, since the inorganic photoconductive material and the inorganic photoreceptor using the same involve many drawbacks, research and development have been progressed for organic photoconductive materials. Further, the organic photoconductive material has been studied and developed generally in recent years and it has been utilized not only for electrostatic recording devices such as the electrophotographic photoreceptor but also has been applied, for example, to sensor devices or organic Electro Luminescent (abbreviated as EL) devices.
The organic photoreceptor using the organic photoconductive material has advantages such that the film formation property for the photosensitive layer is favorable and the flexibility is excellent, as well as it is light in the weight, excellent in the transparency, and a photoreceptor showing good sensitivity to a wavelength region over a wide range can be designed easily by an appropriate sensitizing method. Thus, the organic photoreceptor has been under development as a predominant candidate for the electrophotographic photoreceptor.
In the early days, organic photoreceptors had some drawbacks in point of the sensitivity and the durability thereof, but these drawbacks have been significantly improved by development of a function-separated electrophotographic photoreceptor of which the charge generating function and the charge transporting function are separately attained by different substances. The function-separated photoreceptor of the type has, in addition to the above-mentioned advantages of organic photoreceptors, other advantages in that it has broad latitude in selecting the materials for the photosensitive layer and those having any desired characteristics are relatively readily produced.
The function-separated photoreceptor is grouped into a layered type photoreceptor and a single layer type photoreceptor. In the single layer type function-separated photoreceptor, provided is a single layer type photosensitive layer where a charge generating substance having a charge generating function and a charge transporting substance having a charge transporting function are co-dispersed in a resin called binder resin having a binding function. In the layered type function-separated photoreceptor, provided is a layered type photosensitive layer that comprises a layered type structure of a charge generating layer with a charge generating substance dispersed in a binder resin and a charge transporting layer with a charge transporting substance dispersed in a binder resin.
As the charge generating substance for use in the function-separated photoreceptor, investigated are various materials such as phthalocyanine pigments, squarylium dyes, azo pigments, perylene pigments, polycyclic quinone pigments, cyanine dyes, squaric acid dyes, and pyrylium salt dyes, and various materials having good light resistance and good charge generating capability have been proposed.
Further, various compounds have been known as the charge transporting substance, for example, pyrazoline compounds (for example, refer to Japanese Examined Patent Publication JP-B2 52-4188 (1977)), hydrazone compounds (for example, refer to Japanese Unexamined Patent Publication JP-A 54-150128 (1979), Japanese Examined Patent Publication JP-B2 55-42380 (1980), and Japanese Unexamined Patent Publication JP-A 55-52063 (1980)), triphenylamine compounds (for example, refer to Japanese Examined Patent Publication JP-B2 58-32372 (1983), and Japanese Unexamined Patent Publications JP-A 2-190862 (1990)), and stilbene compounds (for example, Japanese Unexamined Patent Publications JP-A 54-151955 (1979) and JP-A 58-198043 (1983)). In recent years, a compound having a central parent nucleus of a condensed polycyclic hydrocarbon-base substance has been developed, the compound which includes, for example, a pyrene derivative, a naphthalene derivative, and a terphenyl derivative (for example, refer to Japanese Unexamined Patent Publication JP-A 7-48324 (1995)).
The charge transporting substances must satisfy the following requirements:
(1) they are stable to light and heat,
(2) they are stable to active substances such as ozone, nitrogen oxide (chemical formula: NOx) and nitric acid generated by corona discharging in charging the photoreceptor,
(3) they have high charge transporting ability,
(4) they have high compatibility with an organic solvent and a binder resin, and
(5) they can be manufactured easily and inexpensively. However, while the above-stated charge transporting substances can satisfy a portion of the demands but have not yet satisfy all of the demands at high level.
Further, as to characteristics of the photoreceptor, the photoreceptors are required to exhibit good sensitivity even when used in low-temperature environments, and moreover are required to be excellent in environment stability with the characteristics thereof changing in a small range depending on the fluctuation of the ambient environment such as temperature and humidity. However, the charge transporting substances having such characteristics have not yet been obtained.
Further, in recent years, of the above-stated demands, particularly high charge transporting ability has been required for the charge transporting substance. For example, higher sensitivity has been demanded as the photoreceptor characteristics for responding to the requirements of reduction in size and increase in speed of image forming operation to electrophotographic apparatuses such as copying machines and printers, and the charge transporting ability of the charge transporting substance has been demanded to improve as means for attaining higher sensitivity of the photoreceptor.
Further, in the high-speed electrophotographic process, since the time from exposure to the development is short, a photoreceptor of excellent light responsiveness has been required. In a case where the light responsiveness of the photoreceptor is poor, that is, the decaying speed of the surface potential of the photosensitive layer by exposure is slow, the residual potential rises, with the result that the photoreceptor is used repetitively in a state where the surface potential is not decayed sufficiently. Therefore, the surface charges at a portion to be erased are not sufficiently erased by exposure, thereby causing deterioration of the picture quality such as lowering of the image density in an early stage. In the function-separated photoreceptor, the charges generated from the charge generating substance upon light absorption are transported by the charge transporting substance to the surface of the photosensitive layer so that the surface charges of the photosensitive layer at a portion irradiated with a light are eliminated. Therefore, the light responsiveness depends on the charge transporting ability of the charge transporting substance. Accordingly, high charge transporting ability is required for the charge transporting substance also from a viewpoint of attaining a photoreceptor which has high light responsiveness and is capable of forming high quality images even in a high-speed electrophotographic process.
Further, the high durability of electrophotographic apparatus is also demanded. In order to attain the high durability, it is necessary that an electrophotographic photoreceptor has excellent durability and can operate stably for a long period of time. The durability of the photoreceptor is largely influenced by the printing resistance of the outermost layer of the photoreceptor. In a case where a photoreceptor is used being mounted on an electrophotographic apparatus, the outermost layer of the photoreceptor is inevitably scraped at a portion thereof by a contact member such as a cleaning blade or a charge roller. In a case where a scraped amount of the outermost layer of the photoreceptor through the sliding contact, namely an amount of film reduction is large, the electric charge-holding ability of the photoreceptor is lowered, and there arises the problem of the deterioration in image quality. Accordingly, it is demanded for the outermost layer of a photoreceptor to be not readily scraped by the above-stated contact member, that is, to have high printing resistance.
In a case where the charge transporting layer forms the outermost layer of a photoreceptor, as the method of improving the printing resistance of the outermost layer of the photoreceptor, raising the binder resin content has been considered. However, in the case of raising the binder resin content, accordingly the charge transporting substance contained in the charge transporting layer is correspondingly lowered, and this poses the problem that the charge transporting capability of charge transporting layer is lowered and the responsiveness of photoreceptor to light is reduced. Further, from the poor compatibility of a charge transporting substance with a binder resin, the binder resin crystallizes during the preparation of film, and this also consequently poses the problem of causing image defects. Therefore, it has been difficult to realize the photoreceptors not only having good electric properties such as responsibility but also having good durability.
For the purpose of solving the above-mentioned problem, as the charge transporting substance excelling both in charge transporting ability and in compatibility with binder resin, the enamine compounds having enamine structure containing nitrogen atom substituted by different substituents are proposed (refer to Japanese Unexamined Patent Publication JP-A 2004-334125). However, in order to more certainly prevent image defects from being generated, further improvement in the compatibility with the binder resin is also demanded for the enamine compounds disclosed in JP-A 2004-334125.
For the purpose of solving the above-mentioned problem of electrographic photoreceptor, there has been an attempt of lowering the contents of a charge transporting substance by means of giving charge transporting function to a binder resin, and the development has been progressed for the binder resin containing a constituent unit having charge transporting function, that is, so called a photoconductive polymeric material. The specific examples include polycarbonate resin with triarylamine structure in a main chain or a side chain (for example, refer to Japanese Unexamined Patent Publications JP-A 3-221522 (1991), JP-A 4-11627 (1992), JP-A 6-295077 (1994), JP-A 7-258399 (1995) and JP-A 8-62864(1996)); and polyether resin with triarylamine structure in a main chain (for example, refer to Japanese Unexamined Patent Publication JP-A 8-176293 (1996)). These resins are synthesized by homopolymerization or copolymerization of monomers, using as the monomer the compounds having both triarylamine structure and hydroxyl group (for example, refer to Japanese Unexamined Patent Publications JP-A 7-228557 (1995), JP-A 9-194442 (1997), JP-A 2000-136169 and JP-A 2002-249472). However, triarylamine compounds disclosed in, for example, JP-A 7-228557 (1995), JP-A 9-194442 (1997), JP-A 2000-136169) and JP-A 2002-249472, have unsatisfactory charge transporting function, and the resins obtained by polymerization of these triarylamine compounds, which has thus triphenylamine structure, as disclosed in, for example, JP-A 3-221522 (1990), JP-A 4-11627 (1991), JP-A 6-295077 (1994), JP-A 7-258399 (1995), JP-A 8-62864 (1996) and JP-A 8-176293 (1996), have not yet satisfy at high level the demands with regard to charge transporting function and mechanical strength.
For the purpose of solving the above-mentioned problem of the photoconductive polymeric material, bishydroxy-substituted-enamine compounds (hereinafter referred to also as “-bis-hydroxyenamine compound”) that have both enamine structure and two hydroxyl groups, have been proposed as the compounds being useful not only for a raw material compound of polymeric material but also for charge transporting substance by itself (refer to Japanese Unexamined Patent Publications JP-A 2004-269377).
Further, as another means to realize the high durability of photoreceptor, covering the photosensitive layer with a surface protective layer formed of resins or the like is practiced. In the case that the photoreceptor is equipped with a surface protective layer, since the surface protective layer forms the outermost layer, the surface protective layer is demanded to have excellent charge transporting function and excellent wear resistance. As the surface protective layer satisfying above-mentioned demands, the surface protective layer formed of the siloxane type resins containing the structure unit having charge transporting function has been proposed (refer to Japanese Unexamined Patent Publication JP-A 2000-242019).
The bis-hydroxyenamine compounds disclosed in JP-A 2004-269377 have, because of the nitrogen atoms contained in the enamine skeleton, which have the identical substituent groups, a high-symmetry molecule-structure which easily leads crystallization. This consequently poses such problems of poor solubility in solvents and poor compatibility with binder resins. Therefore, for example, in the case of using a bis-hydroxyenamine compound as a charge transporting substance in a charge transporting layer, a part of the compound remains undissolved in the coating solution for forming the layer, and consequently the undissolved part of the compound resides as crystalline in the charge transporting layer, bringing about such bad influences as the formation of image defects and the like. Further, the raw material compounds used for preparing the bis-hydroxyenamine compounds disclosed in JP-A 2004-269377, and an intermediate product generated in the manufacturing process are also easily crystallized, resulting in poor solubility in solvents, and this poses another problem that the reaction does not proceed smoothly. Further, in the case of preparing a polymeric material by using the compounds disclosed in JP-A 2004-269377, there arises the problem that the reaction does not proceed smoothly because of the poor solubility of the compounds in solvents.
From the studies of the inventors, it was found that the solubility of bis-hydroxyenamine compounds disclosed in JP-A 2004-269377 (hereinafter referred to also as “symmetrical bis-hydroxyenamine compound”) in solvents and the compatibility of the compounds with binder resins are higher when the substituent groups substituting for the carbon atoms contained in the enamine skeleton are comparably small groups such as a methyl group, than when the substituent groups are large groups such as an aryl group. Therefore, it is possible to solve, to a certain extent, the above-mentioned problems arisen from the poor solubility in solvents and the poor compatibility with binder resins of the compounds. However, in the case that the substituent group on the enamine moiety of the compounds is an alkyl group, on the contrary, the compounds have some drawbacks that the electric properties, especially hole transporting capability, are inferior to those when the substituent group is an aryl group. Consequently, it has been demanded to enhance the solubility of the compounds in solvents and the compatibility of the compounds with binder resins without deteriorating the electric properties such as the hole transporting capability.
Further, the surface protective layers disclosed in JP-A 2000-242019 have not sufficient charge transporting ability, therefore the surface protective layer being excellent both in charge transporting ability and in mechanical strength has not been realized. Further, as to the photoreceptors disclosed in JP-A 2000-242019, there is incompatibility between the charge transporting substances in charge transporting layers and the structure unit with charge transporting function built in the siloxane type resins forming the surface protective layers. This incompatibility will lead formation of a potential barrier at the interface between the surface protective layer and the charge transporting layer, resulting in the insufficiency of charge injection, and there also arises the problem of decreases in sensitivity and light responsiveness.